In an optical fiber, a transmission loss increases by various external stresses or microbend resulting there from. Therefore, it is necessary to protect an optical fiber against such external stresses and in general, an optical fiber is coated with a coating having a double-layered structure of a soft layer and a hard layer. An inner layer contacting with quartz glass is formed as a buffer layer (hereinafter, referred to as a primary layer) by using a soft resin having comparatively low Young's modulus and an outer layer is formed as a protective layer (hereinafter, referred to as a secondary layer) by using a hard resin having comparatively high Young's modulus. In general, for the primary layer, a resin having Young' modulus of 3 MPa or less is used and for the secondary layer, a resin having Young's modulus of 500 MPa or more is used.
In a method of manufacturing an optical fiber, first, a preform containing quartz glass as a principal component is heated and melted in a drawing furnace to manufacture a quartz glass-made optical fiber. Next, a liquid ultraviolet curing resin is applied to the quartz glass-made optical fiber by using a coating die and then it is irradiated with ultraviolet rays to cure the ultraviolet curing resin. In this way, the quartz glass-made optical fiber is coated with a primary layer and a secondary layer and thus an optical fiber is manufactured. In the present specification, such an optical fiber coated with a primary layer and a secondary layer is referred to as an optical fiber. As described above, by coating the outer circumference of the quartz glass-made optical fiber with a coating resin immediately after the drawing, it is possible to prevent the strength of the optical fiber obtained from being deteriorated.
Furthermore, in the next step, by coating the outer circumference of the optical fiber obtained with a coating layer including a colored resin etc., a colored optical fiber is manufactured. A structure of an optical fiber is shown in FIG. 1. It should be noted that, in the present specification, a plurality of colored optical fibers arranged in the form of a plane and coated all together with a ribbon resin is referred to as an optical fiber ribbon. Furthermore, an optical fiber, a colored optical fiber, and an optical fiber ribbon are referred to all together as an optical fiber.
As a coating resin for an optical fiber, an ultraviolet curing resin is used predominantly. As an ultraviolet curing resin, a urethane acrylate-based resin or an epoxy acrylate-based resin is used.
There is a case where the transmission loss increases when such an optical fiber is immersed in water. In order to obtain an optical fiber having high reliability in which an increase in transmission loss of the optical fiber is suppressed even when used for a long period of time in a state of being immersed in water as described above, various proposals have been made such as improvement of a close adhesion between a primary layer and a glass optical fiber (see Patent Document 1).
In an optical fiber in which the transmission loss has increased, peeling is observed at the interface between the glass optical fiber and the primary layer. The peeling at the interface between the glass optical fiber and the primary layer occurs when a force to peel off the coating layer exceeds an adhesive force at the interface between the glass optical fiber and the coating layer at the interface between the glass optical fiber and the coating layer. If the peeling occurs at the interface, the force applied to the glass becomes uneven, resulting in an increase in transmission loss because of the occurrence of microbend.
The mechanism in which the adhesive force at the interface between the glass optical fiber and the coating layer when being immersed in water is reduced is speculated as follows. When an optical fiber is immersed in water or exposed to a high-humidity atmosphere, the water passes through the coating layer and eventually reaches the interface between the glass optical fiber and the primary layer. There is produced an adhesive force at the interface between the glass optical fiber and the primary layer and in general, the adhesive force is formed by the hydrogen bond between the glass and the functional group in the resin and the chemical bond of an adhesion promoter. However, it is considered that the hydrogen bond is cut when water and the like penetrate the interface between the glass and the primary layer. As described above, it is inferred that the adhesive force at the interface between the glass optical fiber and the primary layer is reduced when the hydrogen bond is cut (see Non-Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-Open Publication No. 1992-268521    [Non-Patent Document 1] N. Akasaka et al., “Design of Optical Fiber Coating”, Proc. of 19th Australian Conference on Optical Fiber Technology (ACOFT), p. 375, 1994